Alcohols, Phenols & Ethers – Comprehensive Study Notes
(Designed for IIT‑JEE / Competitive Exams)
Author: Expert Chemistry Teacher
Date: 18‑Oct‑2026
1. Introduction
| Class | Representative Compounds | Notes |
|---|---|---|
| Alcohols | R–OH (R = aliphatic or aromatic) | - Donor of H via O–H - Acidic (pKa 15–20) |
| Phenols | C6H5–OH | - Stronger acid (pKa ≈ 10) due to resonance stabilization |
| Ethers | R–O–R′ | - Generally unreactive; cleaved under harsh conditions |
Key idea:
The O–H bond is the hallmark of alcohols/phenols; the O–R bonds are the hallmark of ethers.
The electronic nature of the substituent (R) dictates acidity, reactivity, and physical properties.
2. Nomenclature & Classification
2.1 Alcohols
| Type | R‑group | Examples | SN1/SN2 tendency |
|---|---|---|---|
| Primary | R–CH2–OH | ethanol, 1‑butanol | SN2 dominant (except with very bulky reagents) |
| Secondary | R–CH(OH)–R′ | isopropanol | mixture; often SN1 with polar solvents |
| Tertiary | R–C(OH)(R′)(R″) | tert‑butanol | SN1 + E1 preferred |
| Acyclic vs Cyclic | R–CH2–OH vs >CH2O–CH2 | 2‑pentanol vs cyclohexanol | Cyclohexanol (secondary) behaves like secondary |
| Aromatic | Ph–CH2OH (benzyl alcohol) | SN2 (benzylic) |
Important: Primary alcohols can be oxidised to aldehydes (e.g., with PCC) but not to carboxylic acids without over‑oxidation.
2.2 Phenols
| Substituent | Effect on Acidity | Example |
|---|---|---|
| Electron‑donating (–CH3, –OCH3) | Decreases acidity | p‑Methoxy‑phenol |
| Electron‑withdrawing (–NO2, –CN) | Increases acidity | p‑Nitrophenol |
2.3 Ethers
| Type | Representative Reactions | Notes |
|---|---|---|
| Simple (dialkyl) | Williamson synthesis, cleavage (HI) | |
| Alkyl‑aryl | Alkyne → phenol (e.g., via Friedel‑Crafts) | |
| Cyclic | Ether ring cleavage, rearrangements |
3. Physical Properties
| Property | Alcohol | Phenol | Ether |
|---|---|---|---|
| Boiling point | Higher than alkanes (due to H‑bonding) | Highest (strong H‑bonding) | Lower (no H‑bonding) |
| Solubility in water | Good (up to 2–3 carbon atoms) | Good (due to H‑bonding) | Poor (non‑polar) |
| pKa | 15–20 (primary ≈ 16) | ≈10 (aromatic) | > 30 (very weak acid) |
| Flame test | Blue (classical) | Green‑blue (phenolic) | No color change |
4. Acid–Base Behaviour
4.1 Alcohols
[ \mathrm{R!-!OH + B ;\rightleftharpoons; R!-!O^- + H^+} ]
- pKa ≈ 15–20 (primary ≈ 16; secondary ≈ 16.5; tertiary ≈ 17)
- Acidic character increases with increasing electronegativity of R (e.g., CH3 < CH2Cl < CF3).
4.2 Phenols
[ \mathrm{Ar!-!OH + B ;\rightleftharpoons; Ar!-!O^- + H^+} ]
- pKa ≈ 9–10 (e.g., phenol 10.0, p‑nitrophenol 7